1. Field of the Invention
The present invention relates to the field of metal stampers for use in the replication of optically-readable information bearing members. More specifically, the invention is involved in the replication of optically-readable information bearing members by means of an injection molding process utilizing a stamper having a conformal layer of chromium electrolytically deposited over the encoded surface of an existing optically-readable information bearing nickel stamper.
While the improved stamper according to the present invention and the method for making such improved stamper can be used in connection with the production of plastic information bearing surfaces in which micron-sized surface discontinuities are formed in any geometrical configuration, for ease of understanding the devices of the prior art and the invention will be explained using a disc-shaped information bearing member as exemplary. Since the improvement over prior art methods involves the characteristics of the interface between the stamper and the plastic, it will be appreciated that the concepts of the invention broadly apply to the injection molding of information bearing plastic articles of virtually any geometrical shape.
2. Description of the Prior Art
Replication of optically-readable information bearing members by an injection molding process in plastic is well known in the art. Such a process involves liquid plastic injected into a disc-shaped mold and pressed between an encoded stamper surface of nickel (bearing audio, video, and/or digital information) and a rigid backplate. The liquid plastic is allowed to harden and to cool, and is then subsequently "released" from the encoded nickel surface.
Typically, the prior art employs the use of nickel as the stamper material because of its innate structural qualities: a sufficient hardness to bear the fatigue in the mold and release phases of the injection molding process; the considerable ease with which nickel metal can be electrolytically deposited in large amounts; the relatively low cost for the material; and the wide tolerable range of process controls and parameters.
A process for producing a stamper for video-disc purposes is the subject of U.S. Pat. No. 4,211,617, issued July 8, 1980 in the name of Csaba K. Hunyar, assigned to the assignee of the present invention. Hunyar proposes a multi-layer stamper comprised of copper and nickel. The metal layer is applied by vapor bombardment, vapor deposition, or deposition from an electroless plating solution. A "matrix" surface containing the originally recorded information is first silver-plated with an electroless process similar to that used for producing mirror surfaces. The silver film is deposited only to a thickness sufficient to support electroconductivity, typically from about 0.01 to about 2 mils, so that the next electroplating step can be undertaken. The nickel and copper conforming layers are then deposited by an electroplating process to a thickness of from about 3 to 20 mils total thickness, the final layer against which the plastic is molded being the nickel layer.
Upon visual inspection, replicas prepared from a nickel stamper may exhibit glowing orange patches when observed in transmitted light which have the appearance of "surface stains" and which are generally attributed to a surface distortion phenomenon referred to as the "plowing effect" that occurs during the injection molding process. The term "plowing" was coined after observing microphotographs of the surface discontinuities of the replicas and noting that the "bumps" defining the information track thereon were cut away as if by plowing or as if the "bumps" had collided with a sharp object. Subjecting discs prepared from a nickel stamper to . various unique tests (to be described in greater detail hereinafter) indicates a direct correlation between the visibly observable "plowing effect" and disc information reproduction quality.
By monitoring various electronic signals as a laboratory disc test player is operated in modified play and scan modes, it is possible to create maps bearing information related to the extent and intensity of surface defects. Such maps plot defective areas with black dots varying in intensity, determined by the magnitude of the defect. Although such "electronic" testing correlates generally with visual inspection, obviously the "electronic" test results are more representative of actual surface defects. In this description plots or maps which are produced by displaying results of "electronic" testing will be termed "electronic interpretations" as contrasted to, for example, visual observations. Electronic interpretations produced by inspecting replicas prepared in a nickel stamper, exhibit distinct darkened regions in and around the "plowed" areas as confirmed by the above-noted visual inspection. These darkened areas correlate precisely with corresponding regions on the disc that exhibit increased audio noise, increased audio crackle and increased numbers of FM drop-outs. The "audio noise" analysis is performed by analyzing the recovered audio signal in a scan mode of the test disc player; "audio crackle" is an analysis evaluating the audio signal recovered during standard play mode; and "FM drop-outs" involve a measurement of the number of times and position on the disc that the recovered FM signal is interrupted.
Visual and electronic inspection of replicas formed against nickel stampers also reveal the presence of an "orange peel" effect within part of the most strongly "stained" areas. The term "orange peel" is descriptive of the appearance of the outer surface of an information storage disc through which a reading light beam must pass before reaching an information-containing surface. The rough-looking, but uniform, surface defect has the visual appearance of the skin surface of an orange. "Orange peel" causes changes in the refraction coefficient from point to point on the disc surface and results in greater loss of tracking. The possibility has been suggested that audio crackle might arise out of an interaction between the "plowing" and the "orange peel". On close inspection, the "orange peel" effect is seen to be slightly inside the radius of the "stained" or "plowed" region at which audio crackle and FM dropouts are more prominent.
Thus, "orange peel" and "plowing" are deleterious by-products of the replication process for producing optically-readable information bearing members formed with nickel stampers. Accordingly, there is a need in the art for improved audio-visual quality of optically readable information bearing members, and the present invention fills this need in the reduction or elimination of "plowing" and "orange peel" through cleaner release at the interface of the master stamper and the stamped article.